1. Field of the Invention
This invention relates to a lens apparatus, used in combination with a camera that uses an image pickup device, and a camera system that includes this camera and lens apparatus.
2. Description of the Related Art
Since priorly with cameras that use silver halide films and digital cameras that use image pickup devices, systems for correcting the image vibration occurred by a hand shake being applied to a camera have been researched and propositions concerning such systems have been made.
Here, a system for correcting the image vibration due to handshake shall be described briefly.
The hand shake of a camera during the taking of an image is normally a vibration of 1 Hz to 12 Hz, in terms of frequency, and basically in order to enable an image to be taken without image vibration occurring even when such a hand shake is occurring at the time of the taking of the image, the vibration of the camera due to the abovementioned hand shake is detected and an image vibration correction lens is displaced in accordance with a value detected thereby.
Thus in order to enable an image to be taken without causing image vibration even when hand shake occurs, it becomes necessary firstly to accurately detect the vibration of the camera and secondly to correct the change of the optical axis due to the vibration of the camera by displacing an image vibration correction lens.
The detection of this vibration (camera shake) can in principle be performed by equipping a camera with a vibration detection unit, which detects acceleration, velocity, etc., and a computing unit, which electrically or mechanically integrates the output signal of the vibration detection unit and outputs a displacement value.
Image vibration correction can thus be performed by displacing an image vibration correction optical system based on the detected information and controlling the image vibration correction optical unit so as to change the image taking optical axis.
Drive units for driving an image vibration correction optical system include one wherein a coil and a magnet are used, with one of either the coil or magnet being disposed on a fixed part, the other being disposed on an image vibration correction optical system, and the drive being carried out by passing a current through the coil.
In carrying out image vibration correction, vibration in the vertical direction (shall be referred to hereinafter as the “pitch direction”) when the camera is set in the normal position and vibration in the perpendicular, horizontal direction (shall be referred to hereinafter as the “yaw direction”) are detected respectively. Two sets of an abovementioned drive unit are thus disposed to enable image vibration correction in the two directions of yaw and pitch and arrangements are made to enable each of these two drive units to be controlled independently.
Also, methods of reducing the friction in an image vibration correction optical system or drive unit in order to improve the drive characteristics of the drive unit that drives the image vibration correction optical system for correction of image vibration are proposed in Japanese Laid-open Patent No. H09-43657 (U.S. Pat. No. 5,761,546), etc.
With this art, in order to resolve the drive error that results from the friction during driving of the image vibration correction optical system, a high frequency alternating signal of microscopic amplitude is overlaid on the output of the vibration detection unit and the drive for image vibration correction is performed while fluidizing the image vibration correction optical system.
However, with this art, with regard to the optimization of the high frequency microscopic vibration for reduction of friction, only an indication is given concerning the changing of the amplitude of the vibration according to the judgment of whether or not exposure is in progress or changing the amplitude according to the exposure time.
When in a case where an image pickup device is used, such as in a digital camera, the optimal amplitude of the abovementioned high frequency microscopic vibration will differ according to the pixel pitch of the image pickup device. That is, if a high frequency microscopic vibration that is too large in amplitude with respect to the pixel pitch is applied, a vibration will occur in the image that is taken thereby.
Thus with a lens-exchangeable digital camera system, it is difficult to set the same amplitude for the abovementioned high frequency microscopic vibration regardless of the respective pixel pitches of the image pickup devices of the various types of digital cameras onto which a lens apparatus is to be mounted.
Also with a prior-art digital camera, when an optical image of a high frequency no less than the pixel pitch is input, a false signal that does not correspond to the inherent optical image may be generated and lower the image quality as a Moire pattern. Thus in general, an optical low-pass filter is disposed between the image taking optical system and the image pickup device to cut high frequency components of the optical image.
However, the thickness of such an optical low-pass filter presents a large impediment in making a digital camera compact.